Capacitors are elements that are used extensively in semiconductor devices for storing an electrical charge. The capacitors are used in filters, analog-to-digital converters, memory devices, control applications, and many other types of semiconductor devices. One type of the capacitors is a metal-insulator-metal (MIM) capacitor, which is frequently used in mixed signal devices and logic devices, as examples. MIM capacitors are used to store a charge in a variety of semiconductor devices. The MIM capacitors are often used as a storage node in a memory device, for example. A MIM capacitor is typically formed horizontally on a semiconductor wafer, with two metal plates sandwiching a dielectric layer parallel to a surface of the semiconductor wafer.
In general, contact plugs are respectively electrically connected to the two metal plates, but the contact plugs are electrically isolated from each other. In order to form the contact plugs, one of the two metal plates is partially etched and refilled with dielectric material. Then, the contact plugs pass through the dielectric material and are electrically connected to the two metal plates respectively. However, when the metal plate is partially etched, impurities are produced and are residual on a sidewall of the remaining portion of the metal plate. The impurities result in an electron path with a low resistance for electrons when the MIM capacitor is charged, and thus induce a leakage current of the MIM capacitor.